Known conductor bars in the stators of generators have in cross section an internal structure as depicted in FIG. 1 (see also Document DE 19817287). A conductor bar 10 contains a multiplicity of conductor elements 11, which are enclosed by insulation 12. The insulation consists of glass/mica bands, which are wound around and impregnated by a so-called vacuum-pressure method (see also H Sequenz: “Herstellung von Wicklungen electrischer Maschinen” [production of windings of electrical machines] Springer Verlag 1973, pp. 150-154). In the glass/mica bands (13 in FIG. 2), the mica is provided as so-called mica paper 14 which is applied onto a glass filament fabric or glass fabric 15 in order to improve the mechanical strength. Mica is a mineral which belongs to the group of sheet silicates. This sheet-like atomic structure causes mica crystals to have macroscopic shapes which are also very much like platelets.
The mica paper 14 consists of a multiplicity of platelets stacked on and above one another, all of which essentially lie in a plane. The glass/mica bands 13 are wound axially onto the conductor bar in a plurality of layers so that they overlap. Since the electric field is predominantly radial with respect to the bar axis, the platelets are oriented perpendicularly to the field direction. Mica platelets have a very high dielectric strength in this direction, which is then imparted to the insulation as a whole owing to the parallel alignment of the platelets.
That which promotes the dielectric strength, however, is detrimental to the mechanical strength—especially the thermal and mechanical strength: the insulation has a different thermal expansion coefficient to the Cu bar, with the conductor elements 11, which it encloses, so that thermal stresses between the Cu and the insulation 12 are unavoidably formed during thermal cycles. These are greatest in or in the vicinity of the boundary layer. If the band is then wound so that the mica side faces toward the band (which would be favorable in terms of winding technology and for electrical reasons), this can easily lead to mechanical shearing with the first mica layer usually remaining attached to the Cu.
The shearing produces sizeable cavities, which are detrimental for two reasons:                they reduce the thermal conduction radially with respect to the bar direction;        undesired partial discharges are ignited in them.As a simple countermeasure to suppress the cavities, the first layer is often wound with the glass facing downward, and the second and subsequent layers with the mica facing downward.        
In this case, however, two problems arise:                when winding “with the mica outward”, the mica paper 14 is bent sharply over the edges and usually breaks. At least some of the mica platelets therefore lose their alignment, which weakens the dielectric strength.        Between the 1st and 2nd layers, 2 mica layers lie directly on one another, and it is often observed that the insulation then tears between the 1st and 2nd layers, or in the mica of the 1st layer, instead of near the interface between the Cu and the 1st layer.        
A very different approach employs the following measure:                a layer of mica paper is initially wound on the (uncured) green bar with the mica side toward the bar.        A layer of conductive band is wound over this, and is electrically connected to the green bar in the vicinity of the lug holes. Only then is the main insulation applied with the desired thickness. If disbanding now takes place, this will be very likely to occur in the first mica layer directly on the bar. Since this cavity has a floating potential, owing to the conductor band lying above it, partial discharges are prevented.        
This measure, however, does not help to prevent degradation of the thermal conduction. Added to this, another disadvantage is that the additional layer of mica and the conductor band increase the total thickness of the insulation (0.3-0.5 mm on each side).